Apparatus and method for reproducing record for optical recording medium

ABSTRACT

The method for reproducing records for the optical recording medium detects and compensates for detrack, tilt and defocus, having advantages in that: (1) the magnitude and the direction of detrack, tilt and defocus can be detected from a difference signal (for example, a read channel  2  signal or a tracking error signal obtained by processing the read channel  2  signal) between optical reflecting signals detected at the header fields staggered on the basis of the tract center, and compensates for detrack, tilt and defocus, thereby preventing deterioration of data quality caused by detrack, tilt and defocus during a recording/reproducing operation and enabling the stable operation of the system; and (2) the focus servo is rapidly stabilized to enable real-time recording as well as the stable operation of the system.

[0001] This application is a divisional of co-pending application Ser.No. 09/538,748, filed on Mar. 30, 2000, the entire contents of which arehereby incorporated by reference and for which priority is claimed under35 U.S.C. § 120; and this application claims priority of ApplicationNos. 11024/1999 filed in Korea on Mar. 30, 1999, 13569/1999 filed inKorea on Apr. 16, 1999, 14239/1999 filed in Korea on Apr. 21, 1999,18900/1999 filed in Korea on May 25, 1999, 18896/1999 filed in Korea onMay 25, 1999, and 18897/1999 filed in Korea on May 25, 1999 under 35U.S.C. § 119.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a high-density opticalrecording medium system and, more particularly, to an apparatus andmethod for reproducing records for optical recording medium, capable ofdetecting and compensating for detrack, tilt and defocus of the opticalrecording medium.

[0004] 2. Description of the Related Art

[0005] In general, examples of optionally and iteratively rewritableoptical recording medium include rewritable compact disc (CD−RW) andrewritable digital versatile disc (DVD−RW, DVD-RAM, DVD+RW).

[0006] These rewritable optical discs, particularly, DVD-RAMs havesignal tracks made up of lands and grooves and enable the trackingcontrol of an empty disc on which no information signal is written.Recently, information signals are also written on the tracks of landsand grooves so as to enhance recording density. For this purpose, therecent optical pickup for writing and reading information signals usesthe shorter wavelength of laser beam with an increased number ofapertures formed in the object lens and thereby reduces the size of beamfor writing/reading records.

[0007] In order to achieve higher recording density, such a rewritablehigh-density optical disc is designed to have a reduced distance betweenthe signal tracks, i.e., the smaller signal track pitch.

[0008] For the rewritable discs, it is naturally impossible to perform adisc control and a recording operation in an empty disc in which noinformation is written. Thus disc tracks are formed in lands and groovesto write information on, and control information for random access androtation control is separately recorded in the disc, so as to enabletracking control in the empty disc.

[0009] The control information is, as shown in FIG. 1, written on theheader pre-formatted at the beginning position of each sector, or alongthe track in the wobbling profile. The term “wobbling” as used hereinrefers to recording the control information on the boundary of tracks inaccord to variation of laser beam by supplying power of laser diodeswith information for modulating a predetermined clock and applying themodulated clock to the disc, e.g., information about a desired positionand the rotational speed of the disc.

[0010] In a DVD-RAM, the header preformatted at the beginning positionof each sector includes four header fields, e.g., header 1 field, header2 field, header 3 field and header 4 field. Each header field hasvariable frequency oscillator (VFO) areas for generating a referenceclock to acquire bit synchronization of read channels. In the presentinvention, the VFO areas present in the respective header fields (header1 field˜header 4 field) are called VFO1˜VFO4.

[0011] That is, VFO1 and VFO 3 areas are present in the header 1 fieldand the header 3 field, VFO2 and VFO4 areas being in the header 2 fieldand the header 4 field. The VFO1 and VFO3 areas are longer and morestable for signal detection than the VFO2 and VFO4 areas.

[0012] The four header fields are staggered with respect to each otherfrom the track center. FIG. 1 shows an example of the header for thefirst sector in a track. Referring to FIG. 1, the track boundary of theuser area in which data are actually written has a wobbling profile.

[0013] An optical record reproducing apparatus also performs trackingand focus controls with an optical pickup in writing and readinginformation.

[0014] That is, tracking control, e.g., tracking servo involvesdetection of tracking error signals from electrical signals generated inaccordance to the beam trace status and driving a tracking actuator inthe optical pickup based on the tracking error signals to move an objectlens of the optical pickup in the radial direction, thereby changing theposition of the beam to trace a desired track.

[0015] There are some cases where detrack occurs that the beam focus isdeflected from the track center, even though no tracking error signal isdetected. Detrack does not adversely affect the compact discs.

[0016] However, detrack has an adverse effect on the optical discs suchas DVD-RAM where data writing and reading is enabled in both lands andgrooves, because the track pitch is narrowed for purpose of highdensification.

[0017] Due to a depth difference between lands and grooves, detrack mayoccur in the tracks of the grooves even when no detrack is detected inthe tracks of the lands. Likewise, the tracks of the lands may havedetrack while there is no detrack detected in the tracks of the grooves.

[0018] If detrack occurs, writing/reading data becomes harder becausethe beam is ready to shift to the adjacent track to cause a cross talkand clear data from the track.

[0019] In a case where the beam focus is deflected from the disc surfaceduring a focus control, i.e., focus servo, which case will be referredto as “defocus” hereinafter, quality of data deteriorates in writing andreading the data and thereby the system operation becomes unstable.

[0020] The focus servo drives a focus actuator in the optical pickup tomove the optical pickup up or down and make the beam in focus accordingto the turning and up-and-down motions of the optical disc. That is, thefocus actuator drives the object lens for convergence of beam in theupward/downward direction, e.g., in a direction of the focus axis tomaintain a constant distance between the object lens and the opticaldisc.

[0021] However, in the optical discs such as DVD-RAM where data can bewritten in both lands and grooves, the lands and grooves differ in thefocus offset from each other due to a depth difference and cause defocuseven when no focus error signal is detected.

[0022] That is, due to the depth difference between the lands andgrooves, defocus may be detected in the tracks of the grooves even whenno detrack occurs in the tracks of the lands. Likewise, the tracks ofthe lands may have detrack while detrack is not detected in the tracksof the grooves.

[0023] As the defocus status cannot be known only from the focus errorsignals in this case, jitter characteristic deteriorates and the biterror rate (BER) increases. Recording data in this state may result inchange recording characteristics of lands and grooves and hencedeterioration of data quality, which makes the system operationunstable.

[0024] During a resin extracting and hardening process in fabrication ofthe optical disc, distortion may take place in the optical disc andcause eccentricity even when a central aperture is perforated in opticaldisc. Also, deviation of the central aperture causes eccentricityalthough the tracks of the disc are accurately provided in the radialform with a defined pitch. Thus as the disc turns with eccentricity, thecentral axis of the motor is not in perfect accord with the center ofthe track.

[0025] It is thus hard to read out the signals of a desired track only.So, in the CD and DVD systems, a tracking servo is performed accordingto the standards established for the deflected quantity such that thebeam always traces the desired track in spite of eccentricity.

[0026] It means, the tracking servo generates electrical signalscorresponding to the beam trace status and moves the object lens or theoptical pickup body in the radial direction based on the generatedelectrical signals, to change the position of the beam and make the beamtrace the accurate track.

[0027] Meanwhile, the beam can be deflected from a desired track due toa tilt of the disc as well as the eccentricity. This results from amechanic error occurring when the disc is set on a spindle motor. Thatis, the focusing direction is not in perpendicular relation with thetracking direction. This slant state of the disc is called “tilt”.

[0028] Tilt is not so significant for compact discs that have a largetilt margin due to their wide track pitch. The term “tilt margin” asused herein refers to a compensable quantity of tilt of the disc.However, with a growing need of densification of the optical appliancessuch as optical discs, especially in the DVD having the narrower trackpitch, a slight tilt of the disc causes the beam to shift to theadjacent track due to a small radial tilt margin for the jitter. This“detrack” is unavoidable by the tracking servo only. That is, thetracking servo may mistake that the beam is tracing the accurate trackeven when the beam is shifted to the adjacent track due to tilt, whilefocusing on the center of the track.

[0029] This makes it impossible to write/read data in/from a desiredtrack. Thus a double distortion occurs when reading the erroneouslywritten data.

[0030] To cope with this problem, there has been suggested a method inwhich the tilt of the disc can be detected with a dedicated tilt sensor,e.g., a tilt light-receiving device in an optical pickup. However, themethod is not so efficient with a large size of the set.

SUMMARY OF THE INVENTION

[0031] It is, therefore, an object of the present invention to providean apparatus and method for reproducing records for an optical recordingmedium, capable of detecting and compensating for detrack from headerareas staggered with respect to each other.

[0032] It is another object of the present invention to provide anapparatus and method for reproducing records for an optical recordingmedium, capable of detecting and compensating for tilt from header areasstaggered with respect to each other.

[0033] It is further another object of the present invention to providean apparatus and method for reproducing records for an optical recordingmedium, capable of detecting and compensating for defocus from headerareas staggered with respect to each other.

[0034] It is still another object of the present invention to provide anapparatus and method for reproducing records for an optical recordingmedium, capable of iteratively controlling detrack, tilt and defocus ina predefined order.

[0035] To achieve the above objects of the present invention, there isprovided a method for reproducing records for optical recording mediumincludes the steps of: (a) determining a difference between a differencesignal of optical reflecting signals of the optical recording mediumdetected at the non-writable area and the center level at an adjacentdata area to output a first signal; (b) determining a difference betweena difference signal of optical reflecting signals of the opticalrecording medium detected at a second non-writable area and the centerlevel at an adjacent area to output a second signal, the secondnon-writable area being different in phase from the non-writable area;(c) determining a difference between the first signal and the secondsignal to output a variation; (d) comparing the variation with apredetermined threshold, determining that detrack has occurred, if thevariation exceeds the threshold, and outputting the resulting value; and(e) performing a tracking servo based on the resulting value.

[0036] The difference signal between the optical reflecting signalsincludes a read channel 2 signal generated from electrical signalsoutput in proportion to the quantity of beam reflected from the opticalrecording medium.

[0037] A read channel 1 signal means the total output of the split photodetectors. The read channel 2 signal means the differential output ofthe split photo detectors.

[0038] The difference signal between the optical reflecting signalsincludes a tracking error signal obtained by filtering the read channel2 signal generated from electrical signals output in proportion to thequantity of beam reflected from the optical recordng medium.

[0039] The tracking servo step (e) detects the magnitude and thedirection of detrack from the resulting value and the sign of thevariation, respectively.

[0040] The tracking servo step (e) performs the tracking servo in such amanner as to equalize the level of the first signal to the level of thesecond signal.

[0041] The tracking servo step (e) performs the tracking servo in such amanner that two tracking error signals of different phases is insymmetric relation with each other with respect to the center level ofthe adjacent data area.

[0042] In another aspect of the present invention, a method forreproducing records for optical recording medium includes the steps of:(a) determining a difference between a difference signal of opticalreflecting signals of the optical recording medium detected at thenon-writable area and the center level at an adjacent data area tooutput a first signal; (b) determining a difference between a differencesignal of optical reflecting signals of the optical recording mediumdetected at a second non-writable area and the center level of anadjacent data area to output a second signal, the second non-writablearea being different in phase from the non-writable area; (c)determining a difference between the first signal and the second signalto output a variation; (d) comparing the variation with a predeterminedthreshold, determining that tilt has occurred, if the variation exceedsthe threshold, and outputting the resulting value; and (e) performing atilt servo based on the resulting value.

[0043] The tilt servo step (e) detects the magnitude and the directionof tilt from the resulting value and the sign of the variation,respectively.

[0044] The tilt servo step (e) performs the tilt servo in such a manneras to equalize the level of the first signal to the level of the secondsignal.

[0045] The tilt servo step (e) performs the tilt servo in such a mannerthat two tracking error signals of different phases is in symmetricrelation with each other with respect to the center level of theadjacent data area.

[0046] In further another aspect of the present invention, a method forreproducing records for optical recording medium includes the steps of:(a) determining, when no tilt is detected, a first potential differencebetween a ground level and a read channel 2 signal detected at thenon-writable area, and setting the potential difference as a referencevalue; (b) determining, when necessary, a second potential differencebetween a second ground level and a second read channel 2 detected atthe non-writable area, and comparing the first potential difference withthe second potential difference; and (c) determining from the result ofthe comparison in step (b) that tilt has occurred, and performing a tiltservo.

[0047] In still further another aspect of the present invention, amethod for reproducing records for optical recording medium includes thesteps of: (a) determining a difference signal between optical reflectingsignals each detected at the plural non-writable areas of differentphases to output a variation; (b) comparing the variation with apredetermined threshold, determining that defocus has occurred, if thevariation exceeds the threshold, and outputting the resulting value; and(c) performing a focus servo based on the resulting value.

[0048] In the variation outputting step (a), a peak-to-peak voltage ofread channel 1 signals or read channel 2 signals detected at thenon-writable areas is a first signal, and a peak-to-peak voltage of readchannel 1 signals or read channel 2 signals detected at a secondnon-writable areas is a second signal, the second non-writable areasbeing different in phase from the non-writable areas, the variationbeing the difference between the first signal and the second signal.

[0049] The focus servo step (c) is performed in such a manner that thesum signal of the first and second signals is at maximum and thevariation does not exceed the threshold.

[0050] The variation outputting step (a) includes the steps of:determining a potential difference between the tracking error signaldetected at the non-writable area and the track center level of anadjacent data area to output a first signal; determining a potentialdifference between the tracking error signal detected at anothernon-writable area and the track center level of an adjacent data area tooutput a second signal; and determining a difference between the firstand second signals as the variation.

[0051] The focus servo step (c) detects the magnitude and the directionof defocus from the resulting value and the sign of the variation,respectively.

[0052] The focus servo step (c) performs the focus servo in such amanner as to equalize the level of the first signal to the level of thesecond signal.

[0053] In still further another aspect of the present invention, amethod for reproducing records for optical recording medium includes thesteps of: (a) detecting a first variation from a first potentialdifference between a difference signal of optical reflecting signalseach detected at the plural non-writable areas of difference phases anda first reference level, and detecting and compensating detrack of theoptical recording medium from the first variation; (b) detecting asecond variation from a second potential difference between a differencesignal of optical reflecting signals each detected at the pluralnon-writable areas of difference phases and a second reference level,and detecting and compensating tilt of the optical recording medium fromthe second variation; and (c) detecting a third variation from a thirdpotential difference between a difference signal of optical reflectingsignals each detected at the plural non-writable areas of differencephases and a third reference level, and detecting and compensatingdefocus of the optical recording medium from the third variation,wherein the detecting and compensating steps are performed in the orderof detrack, tilt and defocus.

[0054] In still further another aspect of the present invention, anapparatus for reproducing records for optical recording medium includes:a signal generator for generating a difference signal between opticalreflecting signals from electrical signals generated from an opticalpickup for recording/reproducing information on/from the opticalrecording medium; a detrack detector for detecting detrack of theoptical recording medium from a variation of the difference signalbetween the optical reflecting signals of the non-writable areas outputfrom the signal generator, and outputting a detrack error signal; a tiltdetector for detecting tilt of the optical recording medium from avariation of the difference signal between the optical reflectingsignals of the non-writable areas output from the signal generator, andoutputting a tilt error signal; a defocus detector for detecting defocusof the optical recording medium from a variation of the differencesignal between the optical reflecting signals of the non-writable areasoutput from the signal generator, and outputting a defocus error signal;a servo controller for generating a tracking driving signal from thedetrack error signal detected at the detract detector, a tilt drivingsignal from the tilt error signal detected at the tilt detector, and afocus driving signal from the defocus error signal detected at thedefocus detector; a tracking driver for controlling the optical pickupbased on the tracking driving signal to compensate for detrack; a tiltdriver for controlling the optical pickup based on the tilt drivingsignal to compensate for tilt; and a focus driver for controlling theoptical pickup based on the focus driving signal to compensate fordefocus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

[0056]FIG. 1 is a diagram showing a staggered structure of a headerpreformatted at the beginning position of each sector in a generalrewritable disc;

[0057]FIG. 2 is a block diagram showing the structure of an optical discwriting/reading apparatus for controlling detrack, tilt and defocus inaccordance with the present invention;

[0058]FIG. 3 is an exemplary diagram showing an optical detector of theoptical pickup shown in FIG. 2;

[0059]FIG. 4 is an exemplary graph showing read channel 2 signalsdetected at VFO1 and VFO3 areas in the header field depending onvariation of a detrack offset;

[0060]FIGS. 5a-5 c are exemplary diagrams showing the potentialrelationship between the channel 2 signals shown in FIG. 4 and a wobblesignal;

[0061]FIG. 6 is a flowchart of a detrack detecting and compensatingprocedure using read channel 2 signals of the header field in accordancewith the present invention;

[0062]FIG. 7 is an exemplary graph showing tracking error signalsdetected at a header 1,2 field and a header 3,4 field depending onvariation of a detrack offset;

[0063]FIGS. 8a-8 c are exemplary diagrams showing the level variation oftracking error signals detected at a header 1,2 field and a header 3,4field depending on variation of a detrack offset;

[0064]FIG. 9 is a flowchart of a detrack detecting and compensatingprocedure using tracking error signals of the header field in accordancewith the present invention.

[0065]FIG. 10 is an exemplary graph showing read channel 2 signalsdetected at VFO1 and VFO3 areas in the header field depending onvariation of tilt;

[0066]FIGS. 11a-11 c are exemplary diagrams showing the potentialrelationship between the channel 2 signals shown in FIG. 10 and a trackcenter;

[0067]FIG. 12 is a flowchart of a tilt detecting and compensatingprocedure using read channel 2 signals of the header field in accordancewith the present invention;

[0068]FIGS. 13a-13 c are exemplary diagrams showing the relationshipbetween the read channel 2 signals detected at VFO1 and VFO3 areas inthe header field and a ground level, depending on variation of tilt;

[0069]FIG. 14 is an exemplary graph showing tracking error signalsdetected at a header 1,2 field and a header 3,4 field depending onvariation of tilt;

[0070]FIGS. 15a-15 c are exemplary diagrams showing level variation oftracking error signals detected at a header 1,2 field and a header 3,4field depending on variation of tilt;

[0071]FIG. 16 is a flowchart of a tilt detecting and compensatingprocedure using tracking error signals of the header field in accordancewith the present invention;

[0072]FIG. 17 is an exemplary graph showing read channel 2 signalsdetected at VFO1 and VFO3 areas in the header field depending onvariation of a defocus offset;

[0073]FIGS. 18a-18 c are exemplary diagrams showing the level variationof channel 2 signals detected at VFO1 and VFO3 areas in the header fielddepending on variation of a defocus offset;

[0074]FIG. 19 is a flowchart of a defocus detecting and compensatingprocedure using read channel 2 signals of the header field in accordancewith the present invention;

[0075]FIG. 20 is an exemplary graph showing read channel 2 signalsdetected at VFO1 and VFO3 areas in the header field depending onvariation of a defocus offset;

[0076]FIGS. 21a-21 c are exemplary diagrams showing the level variationof read channel 2 signals detected at VFO1 and VFO3 areas in the headerfield depending on variation of a defocus offset;

[0077]FIGS. 22a and 22 b are exemplary diagrams showing the levelvariations of read channel 1 signals and read channel 2 signals detectedat VFO1 and VFO3 areas in the header field depending on variation of adefocus offset;

[0078]FIG. 23 is an exemplary graph showing tracking error signalsdetected at a header 1,2 field and a header 3,4 field depending onvariation of a defocus offset;

[0079]FIGS. 24a-24 c are exemplary diagrams showing level variation ofthe tracking error signals detected at a header 1,2 field and a header3,4 field depending on variation of a defocus offset;

[0080]FIG. 25 is a flowchart of a defocus detecting and compensatingprocedure using tracking error signals of the header field in accordancewith the present invention; and

[0081]FIG. 26 is a flowchart showing a procedure for sequentiallydetecting and compensating for detrack, tilt and defocus of an opticalrecording medium in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0082] A preferred embodiment of the present invention will be describedbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

[0083] The present invention is directed to detection and compensationof detrack, tilt and defocus using variation of a difference signal(e.g., a read channel 2 signal or a tracking error signal obtained fromthe read channel 2 signal processed) between optical reflecting signalsdetected from header fields staggered on the basis of track sectors.

[0084]FIG. 2 is a block diagram showing the structure of an optical discwriting/reading apparatus for reproducing records from an optical discin accordance with the present invention, in which only the principalparts related to detrack, tilt and defocus are shown.

[0085] Referring to FIG. 2, the optical disc writing/reading apparatusincludes: a rewritable optical disc 201; an optical pickup 202 forwriting/reading information on/from the optical disc 201; an RF andservo error generator 203 for generating an RF signal and a servo errorsignal (e.g., tracking error signal, focus error signal, etc.) fromelectrical signals output from the optical pickup 202; a detrackdetector 204 for detecting detrack from the read channel 2 signal or thetracking error signal output from the RF and servo error generator 203;a tilt detector 205 for detecting tilt from the read channel 2 signal orthe tracking error signal; a defocus detector 206 for detecting defocusfrom the read channel 2 signal or the tracking error signal; a servocontroller 207 for generating a tracking driving signal from themagnitude and the direction of detrack detected at the detrack detector204, a tilt driving signal from the magnitude and the direction of tiltdetected at the tilt detector 205, and a focus driving signal from themagnitude and the direction of defocus detected at the defocus detector206; a tracking driver 208 for controlling the optical pickup 202 basedon the tracking driving signal to compensate for the detrack; a tiltdriver 209 for controlling the optical pickup 202 based on the tiltdriving signal to compensate for the tilt; and a focus driver 210 forcontrolling the optical pickup 202 based on the focus driving signal tocompensate for the defocus.

[0086] The optical pickup 202 has an split photo detector for detectingthe quantity of light and converting the detected quantity of light toelectrical signals. The split photo detector can be divided, as shown inFIG. 3, into a predefined number of optical detecting elements, e.g.,four optical detecting elements PDA, PDB, PDC and PDD in the signaltrack direction and the radial direction of the optical disc 201.

[0087] In the present invention as constructed above, the optical disc201 has signal tracks made up of lands and grooves, and data can bewritten/read on/from the tracks of both the lands and the grooves aswell as either the land tracks or the groove tracks. Also, at thebeginning position of each sector, header 1 and 2 fields and header 3and 4 fields are staggered with respect to each other in a free format.That is, the phases of the header 1 and 2 fields are in inverse relationwith those of the header 3 and 4 fields.

[0088] Thus, while setting the optical disc 201, or during thewriting/reading operation, the laser beam emitted from a laser diode ofthe optical pickup 202 is directed onto the signal tracks of the opticaldisc 201 and the beam reflected from the signal tracks of the opticaldisc 201 enters the split photo detector.

[0089] The split photo detector includes a plurality of opticaldetecting elements and outputs to the RF and servo error generator 203electrical signals proportional to the quantity of beam obtained fromthe respective optical detecting elements.

[0090] The optical detector, if constructed as shown in FIG. 3, outputsto the RF and servo error generator 203 electrical signals a, b, c andd, each in proportion to the quantity of beam obtained from therespective optical detecting elements PDA, PDB, PDC and PDD.

[0091] The RF and servo error generator 203 combines the electricalsignals a, b, c and d to generate an RF signal necessary for datareading, and a tracking error signal and a focus error signal, which areall necessary for a servo control. The read channel 1 signal is obtainedby combining the electrical signals a, b, c and d from the split opticaldetector as a+b+c+d, and the read channel 2 signal is obtained bycombining the electrical signals as (a+d)−(b+c). The tracking errorsignal is obtained by processing the read channel 2 signal throughfiltering.

[0092] The split photo detector, if divided into two photodiodes I1 andI2 in the direction of tracks, detects the read channel 1 signal(=I1+I2) and the read channel 2 signal (=I1−I2) from the beam quantitybalance of both photodiodes.

[0093] Here, a wobble signal written on each track as shown in FIG. 1 isdetected only from the read channel 2 signal.

[0094] The present invention involves detection of detrack, tilt anddefocus from variation of a difference signal between optical reflectingsignals detected from header areas staggered on the basis of trackcenter, and compensation for the detected detrack, tilt and defocus.Examples of the difference signal between the optical reflecting signalsinclude a channel 2 signal and a tracking error signal, each of whichwill be separately described later.

[0095] The present invention also provides detection and compensationprocesses for detrack, tilt and defocus, each of which will beseparately described below in the order of detrack, tilt and defocus.

Detrack Detection and Compensation

[0096] Using read channel 2 signal

[0097] The present invention detects detrack using a level differencebetween read channel 2 signals detected at VFO1 and VFO3 areas in theheader field and a wobble signal in the data area. The reason for usingthe signals of VFO1 and VFO3 areas lies in that the VFO1 and VFO3 areasare the longest and most stable areas in the header field and easy todetect.

[0098] For this, among the error signals detected at the RF and servoerror generator 203, read channel 2 signals are input to the detrackdetector 204.

[0099] The levels of the read channel 2 signals detected at the VFO1 andVFO3 areas appear negligible due to variation of the detrack offset withfocus and tracking on, as shown in Table 1. TABLE 1 Detrack Offset [ ]VFO1 [V] VFO3 [ ] 0.00 0.201 0.183 1.00 0.187 0.194 2.00 0.183 0.1903.00 0.192 0.185 4.00 0.183 0.197 5.00 0.183 0.201 6.00 0.187 0.206 7.000.185 0.185 8.00 0.176 0.181 9.00 0.169 0.178 10.00 0.160 0.171

[0100]FIG. 4 is a graph illustrating Table 1, in which the two signalsare almost constant in level (VFO1−VFO3≈0).

[0101] That is, the signal levels are constant within the range ofV_(k−)≦VFO1+VFO3≦V_(k+) irrespective of detrack, for example, whilemoving the track sector.

[0102] As shown in FIGS. 5a-5 c, the center of the wobble signal(hereinafter, referred to as “wobble center”) shifts up/down due tovariation of the detrack offset.

[0103] Thus the quantity (=magnitude) and the direction of the detrackcan be determined from comparison of the potential difference betweenthe wobble center of the read channel 2 and the VFO1 signal (VFO1potential−wobble center potential=Vpp11) with the potential differencebetween the wobble center and the VFO3 signal (VFO3 potential−wobblecenter potential=Vpp12).

[0104] That is, FIGS. 5a-5 c are exemplary diagrams showing read channel2 signals detected under variation of the detrack offset with the focusand tracking on when tilt is zero (=mechanism 0). Referring to FIGS.5a-5 c, the signal on the above and left side is the read channel 2signal detected at the VFO area of the header 1,2 field and, in theinverse phase, the signal on the below and right side is the readchannel 2 signal detected at the VFO area of the header 3,4 field.

[0105] In the present invention, the wobble center shifts depending onthe detrack offset and thereby a voltage V_(WC) detected at the wobblecenter changes. Thus the voltages detected at VFO1 and VFO3 are used asreference levels.

[0106] In a case where there is no detrack, i.e., the beam is correctlyfocused on the track center, the potential difference between the wobblecenter and the VFO1 signal (Vpp11=V_(VFO1)−V_(WC)) is almost equal tothe potential difference between the wobble center and the VFO3 signal(Vpp12=V_(VFO3)−V_(WC)), as shown in FIG. 5b.

[0107] This can be expressed by Equation 1.

V _(VFO1) −V _(WC) ≈V _(VFO3) −V _(WC)  [Equation 1]

[0108] The value V_(VFO1) (or V_(VFO3)) is determined while holding thepeak and the bottom of the VFO1 (or VFO3) signal and then compared withthe voltage of the wobble center. Alternatively, the value V_(VFO1) (orV_(VFO3)) is determined while holding the center of the VFO1 (or VFO3)signal and then compared with the voltage of the wobble center.

[0109] According to the present invention, if the potential differenceVpp11 between the VFO1 signal and the wobble center is not equal to thepotential difference Vpp12 between the VFO2 signal and the wobblecenter, i.e., the absolute value of the difference between the twopotential differences (=Vpp11−Vpp12) exceeds a threshold V_(Th1), it isdetermined that detrack has occurred; otherwise, the beam is determinedas correctly focused on the track center, that is, “on track”.

|Vpp 11−Vpp 12|≦V _(Th1)  [Equation 2]

[0110] As the values of the read channel 2 signals detected at theheader 1,2 field and the header 3,4 field are variable depending on thedisc, the ratio of the two signals is normalized as expressed byEquation 3. $\begin{matrix}{{\frac{{Vpp11} - {Vpp12}}{{Vpp11} + {Vpp12}}} \leq V_{Th1}} & \left\lbrack {{Equation}\quad 3} \right\rbrack\end{matrix}$

[0111] In a case where it is determined that detrack has occurred as theabsolute value of the difference between the two potentials Vpp11 andVpp12 is larger than the threshold V_(Th1), the magnitude and thedirection of detrack are be detected from the absolute value and thesign of the potential difference, respectively.

[0112] That is, when the value (=Vpp11−Vpp12) is ΔV1 and the absolutevalue of ΔV1 is greater than the threshold V_(Th1), the magnitude andthe direction of detrack can be known from the value and the sign ofΔV1, respectively.

[0113] If the sign of ΔV1 is negative (−), detrack is to be compensatedby ΔV1 in the positive (+) direction; otherwise, if the sign of ΔV1 ispositive (+), detrack is to be compensated by ΔV1 in the negative (−)direction. Therefore, compensation for detrack has to be performed insuch a direction as to equalize the two potential differences Vpp11 andVpp 12.

[0114] In connection with this, the detrack detector 204 outputs to theservo controller 207 detrack error signals indicating the magnitude andthe direction of detrack, which correspond to the absolute value and thesign of ΔV1, respectively. The servo controller 207 converts the detrackerror signals to a tracking driving signal and outputs the trackingdriving signal to the tracking driver 208.

[0115] The tracking driver 208 drives a tracking actuator in the opticalpickup 202 based on the tracking driving signal, i.e., moves the opticalpickup 202 by the magnitude of detrack in the positive (+) or negative(−) direction such that the optical pickup 202 lies in accord with thetrack center line of the optical disc 201.

[0116]FIG. 6 is a flowchart of the above procedure.

[0117] Referring to FIG. 6, the step 301 sets an initial detrack offsetDT0, a threshold V_(Th1) and a detrack limit iteration number NL1. Thestep 302 measures Vpp11 and Vpp12 at detrack offset DT0, thencalculating the difference between Vpp11 and Vpp12, i.e., ΔV1(=Vpp11−Vpp12). The step 303 determines whether the absolute value ofΔV1 exceeds the threshold V_(Th1) preset in the step 301. The reason forusing the absolute value of ΔV1 lies in that ΔV1 can be a positive (+)or negative (−) value. If the absolute value of ΔV1 is smaller than orequal to the threshold V_(Th1) in the step 303, which means that nodetrack occurs, i.e., the beam is correctly focused on the track center,the routine is terminated; otherwise, if the absolute value of ΔV1 islarger than the threshold V_(Th1), which means that detrack occurs, thestep 304 determines the direction of detrack from a judgment of whetherthe sign of ΔV1 is positive (+) or negative (−). With negative (−) signof ΔV1, the step 305 increments the detrack offset DT0; otherwise, withpositive (+) sign of ΔV1, the step 306 measures again the variation ΔV1of the read channel 2 signals detected at the header field. Thereafter,the step 307 compares the absolute value of the variation ΔV1 with thethreshold V_(Th1). If the absolute value of ΔV1 is smaller than or equalto the threshold V_(Th1), the routine is terminated; otherwise, if theabsolute value of ΔV1 is larger than the threshold V_(Th1), which meansthat detrack is not completely compensated, the procedure proceeds tothe step 308. The step 308 applies the current loop iteration number tothe detrack iteration number and the step 309 compares the detrackiteration number with the preset detrack limit iteration number NL1.

[0118] If the detrack iteration number is larger than or equal to thepreset detrack limit iteration number NL1, the routine terminates;otherwise, the procedure returns to the step 305.

[0119] This comparison of the detrack iteration number with the detracklimit iteration number is to prevent a possible case where the variationΔV1 exceeds the threshold V_(Th1), i.e., no convergence occurs. In sucha case, the routine provides an unlimited loop, resets the detrack limititeration number and compares the detrack limit iteration number withthe detrack iteration number.

[0120] Meanwhile, if the step 304 determines that the sign of ΔV1 ispositive (+), the detrack offset DT0 is decremented in step 310 and thevariation ΔV1 of the read channel 2 signals detected at the header fieldis measured again in step 311. The step 312 compares the absolute valueof the variation ΔV1 with the threshold V_(Th1). If the absolute valueof ΔV1 is smaller than or equal to the threshold V_(Th1), the routine isterminated; otherwise, if the absolute value of ΔV1 is larger than thethreshold V_(Th1), which means that detrack is not completelycompensated, the procedure proceeds to the step 313. The step 313applies the current loop iteration number to the detrack iterationnumber and the step 314 compares the detrack iteration number with thepreset detrack limit iteration number NL1. If the detrack iterationnumber is larger than or equal to the detrack limit iteration number,the routine terminates; otherwise, the procedure returns to the step310.

[0121] 2) Using tracking error signal

[0122] The present invention detects detrack using a level differencebetween tracking channel 2 signals detected at the header 1,2 field andthe header 3,4 field staggered with respect to each other and areference signal. The reference signal level is the center level of thetracking error signal detected at a user area.

[0123] For this, the tracking error (TE) signals among the servo errorsignals detected at the RF and servo error generator 203 are input tothe detrack detector 204. The TE signals can be obtained by filteringthe read channel 2 signals, for example, using a low-pass filter.

[0124] After sampling the tracking error signals output from the header1,2 field and the header 3,4 field, the detrack detector 204 detects thelevel difference between the tracking error signals and the referencesignal

[0125] Table 2 shows the tracking error signal levels under bestconditions for generating the servo error signals while controllingdefocus and detrack in a state of tilt zero (i.e., mechanism 0), inwhich the tracking error signal levels vary depending on variation ofthe detrack offset at fixed tilt and defocus offsets. TABLE 2 DetrackOffset [ ] Header 1, 2 [V] Header 3, 4 [V] 0.00 4.90 0.50 1.00 4.30 1.302.00 4.10 1.50 3.00 3.70 2.10 4.00 3.20 2.50 5.00 2.50 3.10 6.00 1.704.00 7.00 0.80 4.30 8.00 0.20 4.80 9.00 0.00 4.90 10.00 −1.10 5.50

[0126]FIG. 7 is a graph illustrating Table 2, in which no detrack isdetected when the potential difference between the tracking error signaldetected at the header 1,2 field and the reference signal is insymmetric relation with the potential difference between the trackingerror signal detected at the header 3,4 field and the reference signal.

[0127] That is, the tracking error signals are significantly shifted upand down in the header field. For tracking error signals detected at theuser area on which data is actually written, the two potentialdifferences are almost equal to each other when no detrack occurs, i.e.,the beam is focused on the track center, whereas they are not equal toeach other when detrack occurs, i.e., the beam passes through the header1,2 field and the header 3,4 field.

[0128] Thus whether or not detrack has occurred can be determined bycomparing the potential difference between the tracking error signal atthe header 1,2 field and the reference signal (tracking error signalpotential at header 1,2 field−reference potential=Vp11) with thepotential difference between the tracking error signal at the header 3,4field and the reference signal (tracking error signal potential atheader 3,4 field−reference potential=Vp12).

[0129]FIGS. 8a-8 c are exemplary diagrams showing tracking error signalsvarying depending on variation of the detrack offset with tracking andfocus on at tilt=0.

[0130] Referring FIGS. 8a-8 c, the left-hand signal is the trackingerror signal V_(HD12) detected at the header 1,2 field, the right-handsignal being the tracking error signal V_(HD34). A voltage V_(TE)detected at the center level of the tracking error signal at the userarea is preferably the voltage of the reference level.

[0131] In a case where no detrack occurs, the potential differencebetween the tracking error signal at the header 1,2 field and thereference level (VP11=|V_(HD12)−V_(TE)|) is almost equal to thepotential difference between the tracking error signal at the header 3,4field and the reference level (Vp12=|V_(HD34)−V_(TE)|), as shown in FIG.8b. That is, the potential difference Vp11=|V_(HD12)−V_(TE)| is insymmetric relation with the potential difference Vp12=|V_(HD34)−V_(TE)|.

[0132] This can be expressed by Equation 4.

|V _(HD12) −V _(TE) |≈|V _(HD34) −V _(TE)|  [Equation 4]

[0133] It is determined that no detrack has occurred, when the potentialdifference Vp11 between the tracking error signal at the header 1,2field and the reference level is not equal to the potential differenceVp12 between the tracking error signal at the header 3,4 field and thereference level, as shown in FIGS. 8a and 8 c, i.e., the potentialdifference Vp11 is in asymmetric relation with to the potentialdifference Vp12. The asymmetry increases with greater magnitude ofdetrack.

[0134] The can be expressed by Equation 5.

|V _(HD12) −V _(TE) |≠|V _(HD34) −V _(TE)|  [Equation 5]

[0135] It is determined that detrack occurs, when the potentialdifference between the tracking error signal at the header 1,2 field andthe reference level (VP11=|V_(HD12)−V_(TE)|) is not equal to thepotential difference between the tracking error signal at the header 3,4field and the reference level (Vp12=|V_(HD34)−V_(TE)|), i.e., theabsolute value of the difference between the two potentials (=Vp11−Vp12)exceeds a threshold V_(Th2). Otherwise, as expressed by Equation 6, itis determined as “on-track” where the beam is correctly focused on thetrack center.

|Vp 11−Vp 12|≦V _(Th2)  [Equation 6]

[0136] As such, after calculation of the potential difference Vp11between the tracking error signal at the header 1,2 field and thereference level, and the potential difference Vp12 between the trackingerror signal at the header 3,4 field and the reference level, thedifference between the two potential differences Vp11 and Vp12 iscompared with the threshold, as a result of which the magnitude and thedirection of detrack are detected.

[0137] When the difference between the two potential differences(Vp11−Vp12) is ΔV2, the absolute value of ΔV2 indicates the magnitude ofdetrack, the sign of ΔV2 indicating the direction of detrack. That is,it is detected in which direction the optical disc is deflected from thetrack center with respect to a normal state.

[0138] If the sign of ΔV2 is negative (−), detrack is to be compensatedby ΔV1 in the positive (+) direction; otherwise, if the sign of ΔV2 ispositive (+), detrack is to be compensated by ΔV2 in the negative (−)direction. That is, compensation for detrack has to be performed in sucha direction as to equalize the two potential differences Vp11 and Vp12,i.e., the value ΔV2 becomes zero.

[0139] Because the values of the tracking error signals detected at theheader 1,2 field and the header 3,4 field are variable depending on thedisc, the ratio of the two signals is normalized as expressed byEquation 7. $\begin{matrix}{{\frac{{Vp11} - {Vp12}}{{Vp11} + {Vp12}}} \prec V_{Th2}} & \left\lbrack {{Equation}\quad 7} \right\rbrack\end{matrix}$

[0140] That is, if the Equation 7 is satisfied, it is determined that nodetrack has occurred, otherwise, it means that detrack has occurred,after which the magnitude and the direction of detrack are detected fromthe absolute value and the sign of (Vp11−Vp12), respectively.

[0141] In connection with this, the detrack detector 204 calculates ΔV2in the above-described manner and outputs to the servo controller 207detrack error signals indicating the magnitude and the direction ofdetrack, which correspond to the absolute value and the sign of ΔV2,respectively. The servo controller 207 converts the detrack errorsignals to a tracking driving signal and outputs the tracking drivingsignal to the tracking driver 208. That is, the servo controller 207generates a tracking driving signal such that the potential differencebetween the tracking error signal at the header 1,2 field and thereference level (Vp11=|V_(HD12)−V_(TE)|) is in symmetric relation withthe potential difference between the tracking error signal at the header3,4 field and the reference level (Vp12=|V_(HD34)−V_(TE)|), and outputsthe tracking driving signal to the tracking driver 208.

[0142] The tracking driver 208 moves a tracking actuator in the opticalpickup 202 based on the tracking error signal, i.e., by the magnitude ofdetrack in the positive (+) or negative (−) direction such that theoptical pickup 202 lies in accord with the track center line of theoptical disc 201.

[0143]FIG. 9 is a flowchart of the above procedure.

[0144] Referring to FIG. 9, the step 401 sets an initial detrack offsetDT0, a threshold V_(Th2) and a detrack limit iteration number NL1.

[0145] The step 402 measures Vp11 and Vp12 at detrack offset DT0, thencalculating the difference between Vp11 and Vp12, i.e., ΔV2(=Vp11−Vp12). The step 403 determines whether the absolute value of ΔV2exceeds the threshold V_(Th2) preset in the step 401. If the absolutevalue of ΔV2 is smaller than or equal to the threshold V_(Th2) in thestep 403, which means “on-track”, i.e., the beam is correctly focused onthe track center, the routine is terminated; otherwise, if the absolutevalue of ΔV2 is larger than the threshold V_(Th2), which means thatdetrack occurs, the step 404 determines the direction of detrack from ajudgment of whether the sign of ΔV2 is positive (+) or negative (−).With the negative (−) sign of ΔV2, the step 405 increments the detrackoffset DT0 and the step 406 measures again the variation ΔV2 of thetracking error signals detected at the header 1,2 field and the header3,4 field. Then, the step 407 compares the absolute value of thevariation ΔV2 with the threshold V_(Th2). If the absolute value of ΔV2is smaller than or equal to the threshold V_(Th2), the routine isterminated; otherwise, if the absolute value of ΔV2 is larger than thethreshold V_(Th2), which means that detrack is not completelycompensated, the procedure proceeds to the step 408. The step 408applies the current loop iteration number to the detrack iterationnumber and the step 409 compares the detrack iteration number with thepreset detrack limit iteration number NL1. If the detrack iterationnumber is larger than or equal to the preset detrack limit iterationnumber NL1, the routine terminates; otherwise, the procedure returns tothe step 405.

[0146] Meanwhile, if the sign of ΔV2 is positive (+) in step 404, thestep 410 decrements detrack offset DT0 and the above process is repeatedin steps 411 to 414.

[0147] The present invention presets the thresholds and reduces timerequired for detecting and compensating detrack during the actual datawrite operation, thereby enabling a real time write operation throughrapid stabilization of tracking servo.

Tilt Detection and Compensation

[0148] Using read channel 2 signal

[0149] The track center value is best detected from wobble signals,since the wobble signals are formed regularly along the track boundaryduring fabrication of the disc and tilt does not affect the center ofthe wobble signals.

[0150] Thus the present invention detects tilt using a level differencebetween VFO1 and VFO3 signals at the header field of read channel 2 andthe track center.

[0151] For this, among error signals detected at the RF and servo errorgenerator 203, read channel 2 signals are input to the tilt detector205.

[0152] The levels of read channel 2 signals detected at VFO1 and VFO3areas appear negligible due to variation of tilt with focus and trackingon, as shown in Table 3. TABLE 3 Radial Tilt [°] VFO1 [V] VFO3 [V] −1.00.201 0.178 −0.8 0.215 0.183 −0.6 0.210 0.187 −0.4 0.197 0.187 −0.20.197 0.201 0.0 0.192 0.206 0.2 0.210 0.178 0.4 0.151 0.224 0.6 0.1270.219 0.8 0.110 0.215 1.0 0.114 0.197

[0153]FIG. 10 is a graph illustrating Table 3, in which the two signalsare almost constant in level (VFO1−VFO3≈0).

[0154] That is, the signal levels are constant within the range ofV_(k−)≦VFO1+VFO3≦V_(k+) irrespective of tilt.

[0155] As shown in FIGS. 11a-11 c, the VFO1 and VFO3 signals are shiftedabove/below the track center due to variation of tilt. However, tiltdoes not affect the track center.

[0156] Thus the quantity (=magnitude) and the direction of the tilt canbe detected from comparison of the potential difference between thetrack center and the VFO1 signal of read channel 2 (VFO1 potential−trackcenter potential=Vpp21) with the potential difference between the trackcenter and the VFO3 signal of read channel 2 (VFO3 potential−trackcenter potential=Vpp22).

[0157] That is, FIGS. 11a-11 c are exemplary diagrams showing readchannel 2 signals detected under variation of tilt with the focus andtracking on. Referring to FIGS. 11a-11 c, the left-hand signal is theread channel 2 signal detected at the VFO1 area of the header field andthe right-hand signal is the read channel 2 signal detected at the VFO3area of the header field. The voltage V_(WC) detected at the trackcenter is the reference voltage.

[0158] In a case where tilt is zero, i.e., there is no radial tilt, thepotential difference between the track center and the VFO1 signal(Vpp21=V_(VFO1)−V_(WC)) is almost equal to the potential differencebetween the track center and the VFO3 signal (Vpp22=V_(VFO3)−V_(WC)), asshown in FIG. 11b. This means, the potential difference Vpp21 is insymmetric relation with the potential difference Vpp22.

[0159] This can be expressed by Equation 8.

V _(VFO1) −V _(WC) ≈V _(VFO3) V _(WC)  [Equation 8]

[0160] The value V_(VFO1) (or V_(VFO3)) is determined while holding thepeak and the bottom of the VFO1 (or VFO3) signal and then compared withthe voltage of the track center. Alternatively, The value V_(VFO1) (orV_(VFO3)) is determined while holding the center of the VFO1 (or VFO3)signal and then compared with the voltage of the track center.

[0161] If the potential difference Vpp21 between the VFO1 signal and thetrack center is not equal to the potential difference Vpp22 between theVFO2 signal and the track center, i.e., Vpp21 is in asymmetric relationwith Vpp22, it means that tilt occurs.

[0162] For example, as shown in FIG. 11a, when V_(VFO1)−V_(WC)

V_(VFO3)−V_(WC), i.e., Vpp21>Vpp22, it means that tilt of about 1°occurs; and as shown in FIG. 11c, when V_(VFO1)−V_(WC)

V_(VFO3)−V_(WC), i.e., Vpp21<Vpp22, it means that tilt of about −1°occurs

[0163] As such, the magnitude and the direction of tilt can be detectedfrom calculation of the potential difference Vpp21 between the trackcenter and the VFO1 signal and the potential difference Vpp22 betweenthe track center and the VFO2 signal, and comparison of the twopotential differences.

[0164] When the value (=Vpp21−Vpp22) is ΔV3 and the absolute value ofΔV3, the magnitude and the direction of tilt are detected from the valueand the sign of ΔV3, respectively. That is, it can be known whether thedisc is bending up or down with respect to a normal state.

[0165] If the sign of ΔV3 is negative (−), tilt is to be compensated byΔV3 in the positive (+) direction; otherwise, if the sign of ΔV3 ispositive (+), tilt is to be compensated by ΔV3 in the negative (−)direction. Therefore, compensation for tilt has to be performed in sucha direction as to equalize the two potential differences Vpp21 andVpp22.

[0166] In connection with this, the tilt detector 205 outputs to theservo controller 207 tilt error signals indicating the magnitude and thedirection of tilt, which correspond to the absolute value and the signof ΔV3, respectively. The servo controller 207 converts the tilt errorsignals to a tilt driving signal and outputs the tilt driving signal tothe tilt driver 209.

[0167] The tilt driver 209 moves the disc or the optical pickup fordirect control of tilt based on the tilt driving signal, i.e., by themagnitude of tilt in the positive (+) or negative (−) direction.

[0168]FIG. 12 is a flowchart of the above procedure.

[0169] Referring to FIG. 12, the step 501 sets an initial tilt offsetT0, a threshold V_(Th3) and a tilt limit iteration number NL2. The step502 measures Vpp21 and Vpp22 at tilt offset T0, then calculating thedifference between Vpp21 and Vpp22, i.e., ΔV3 (=Vpp21−Vpp22). The step503 determines whether the absolute value of ΔV3 exceeds the thresholdV_(Th3) preset in the step 501. If the absolute value of ΔV3 is smallerthan or equal to the threshold V_(Th3) in the step 503, which means thatno tilt occurs, the routine is terminated; otherwise, if the absolutevalue of ΔV3 is larger than the threshold V_(Th3), which means that tiltoccurs, the step 504 determines the direction of tilt from a judgment ofwhether the sign of ΔV3 is positive (+) or negative (−).

[0170] With negative (−) sign of ΔV3, the step 505 increments the tiltoffset T0 and the step 506 measures again the variation ΔV3 of the readchannel 2 signals detected at the VFO1 and VFO3 areas in the headerfield. Then, the step 507 compares the absolute value of the variationΔV3 with the threshold V_(Th3). If the absolute value of ΔV3 is smallerthan or equal to the threshold V_(Th3), the routine is terminated;otherwise, if the absolute value of ΔV3 is larger than the thresholdV_(Th3), which means that tilt is not completely compensated, theprocedure proceeds to the step 508. The step 508 applies the currentloop iteration number to the tilt iteration number and the step 509compares the tilt iteration number with the preset tilt limit iterationnumber NL2. If the tilt iteration number is larger than or equal to thepreset tilt limit iteration number NL2, the routine terminates;otherwise, the procedure returns to the step 505.

[0171] Meanwhile, if the step 504 determines that the sign of ΔV3 ispositive (+), the tilt offset T0 is decremented in step 510 and theabove process is repeated in steps 511 to 514. The determination andcomparison process for the tilt limit iteration number NL2 is the samein reason as the previously described process for the detrack limititeration number NL1.

[0172] The present invention can detect the magnitude and the directionof tilt by using a ground level (also, called “initial level”) ratherthan the track center as the reference value for detecting tilt.

[0173] For this, the present invention can use either one of VFO1 andVFO3 signals of read channel 2 in the header field as well as both ofthem. Also, the present invention can use a voltage V_(BOT1) or V_(BOT2)while holding the bottom of read channel 2 signals in the header field,or a voltage V_(TOP1) or V_(TOP2) while holding the top of the readchannel 2 signals.

[0174] For example, it is supposed that use is made of a read channel 2signal at the VFO1 area in the header field, particularly, a voltageobtained by holding the top of the read channel 2 signal at the VFO1area.

[0175] First, a potential difference V_(TOP1) is measured between theground potential when no tilt occurs and the top signal of the readchannel 2 signal at the VFO1 area. If the potential difference V_(TOP1)is measured when no tilt occurs, as shown in FIG. 13b, tilt is detectedwith the potential difference used as a preset threshold.

[0176] That is, if the potential difference between the ground potentialand the top signal of the read channel 2 signal at the VFO1 is not equalto the potential difference measured when no tilt occurs, it means thattilt has occurred.

[0177] It is determined that tilt has occurred, for example, if thepotential difference V′_(TOP1) between the ground potential and the topsignal of the read channel 2 signal at the VFO1 area is higher than thepotential difference V_(TOP1) measured when no tilt occurs, as shown inFIG. 13a, or if the potential difference V″_(TOP1) between the groundpotential and the top signal of the read channel 2 signal at the VFO1area is lower than the potential difference V_(TOP1) measured when notilt occurs, as shown in FIG. 13c.

[0178] The direction of tilt can be known from the sign of thedifference between the two potential differences. That is, the magnitudeand the direction of tilt can be detected from the absolute value andthe sign of the difference between the two potential differences.

[0179] For example, when V_(TOP1)−V″_(TOP1)=+ΔV3′, which means that tilthas occurred by ΔV3′, as shown in FIG. 13c, compensation for tilt has tobe performed by ΔV3′ in the negative (−) direction.

[0180] The present invention also uses the potential difference V_(BOT1)as a reference value at the time when no tilt occurs and detects themagnitude and the direction of tilt using the relationship between thereference value and the potential difference between the groundpotential and the bottom signal of the read channel 2 signal at the VFO1area. Likewise, the present invention uses V_(TOP2) and V_(BOT2).

[0181] Such as in the present invention, during tilt or servo control,the quantity of tilt between the optical axis and the disc plane can bedetected and controlled by any one of the above-stated methods.

[0182] 2) Using tracking error signal

[0183] The present invention detects tilt using a level differencebetween tracking error signals detected at the header 1,2 field and theheader 3,4 field staggered with respect to each other and a referencesignal. The reference signal level is the center level of the trackingerror signal detected at a user area.

[0184] For this, the tracking error (TE) signals among the servo errorsignals detected at the RF and servo error generator 203 are input tothe tilt detector 205

[0185] After sampling the tracking error signals output from the header1,2 field and the header 3,4 field, the tilt detector 205 detects thelevel difference between the tracking error signals and the referencesignal

[0186] Table 4 shows the tracking error signal levels detected whilevarying tilt at fixed detrack and defocus offsets with focus andtracking on. TABLE 4 Radial Tilt [°] Header 1, 2 [V] Header 3, 4 [V]−1.0 0.70 3.30 −0.8 1.00 3.50 −0.6 1.50 3.50 −0.4 1.80 3.70 −0.2 2.203.40 0.0 2.20 3.10 0.2 2.40 3.10 0.4 2.40 2.90 0.6 2.40 2.60 0.8 2.202.40 1.0 2.10 1.80

[0187]FIG. 14 is a graph illustrating Table 4, in which it means that notilt has occurred when the potential difference between the trackingerror signal detected at the header 1,2 field and the reference signalis in symmetric relation with the potential difference between thetracking error signal detected at the header 3,4 field and the referencesignal.

[0188] That is, the tracking error signals are significantly shifted upand down in the header field. For tracking error signals detected at theuser area on which data is actually written, the two potentialdifferences are almost equal to each other when no tilt occurs, i.e.,the beam is at the track center, whereas they are not equal to eachother when tilt occurs, i.e., the beam passes through the header 1,2field and the header 3,4 field.

[0189] Thus whether tilt has occurred or not can be determined bycomparing the potential difference between the tracking error signal atthe header 1,2 field and the reference signal (tracking error signalpotential at header 1,2 field−reference potential=Vp21) with thepotential difference between the tracking error signal at the header 3,4field and the reference signal (tracking error signal potential atheader 3,4 field−reference potential=Vp22).

[0190]FIGS. 15a-15 c are exemplary diagrams showing tracking errorsignals varying depending on variation of the tilt offset with trackingand focus on.

[0191] Referring FIGS. 15a-15 c, the left-hand signal is the trackingerror signal V_(HD12) detected at the header 1,2 field, the right-handsignal being the tracking error signal V_(HD34). A voltage V_(TE)detected at the center level of the tracking error signal at the userarea is preferably the voltage of the reference level.

[0192] In a case where no tilt occurs, the potential difference betweenthe tracking error signal at the header 1,2 field and the referencelevel (Vp21=|V_(HD12)−V_(TE)|) is almost equal to the potentialdifference between the tracking error signal at the header 3,4 field andthe reference level (Vp22=|V_(HD34)−V_(TE)|), as shown in FIG. 15b. Thatis, the potential difference Vp21=|V_(HD12)−V_(TE)| is in symmetricrelation with the potential difference Vp22=|V_(HD34)−V_(TE)|.

[0193] This can be expressed by Equation 9.

|V _(HD12) −V _(TE) ≈|V _(HD34) −V _(TE)|  [Equation 9]

[0194] It is determined that no tilt has occurred, when the potentialdifference Vp21 between the tracking error signal at the header 1,2field and the reference level is not equal to the potential differenceVp22 between the tracking error signal at the header 3,4 field and thereference level, as shown in FIGS. 15a and 15 c, i.e., the potentialdifference Vp21 is in asymmetric relation with to the potentialdifference Vp22. The asymmetry increases with greater magnitude of tilt.

[0195] The can be expressed by Equation 10.

|V _(HD12) −V _(TE) |≠|V _(HD34) −V _(TE|)  [Equation 10]

[0196] It is determined that tilt has occurred, when the potentialdifference between the tracking error signal at the header 1,2 field andthe reference level (Vp21=|V_(HD12)−V_(TE)|) is not equal to thepotential difference between the tracking error signal at the header 3,4field and the reference level (Vp22=|V_(HD34)−V_(TE)|), i.e., theabsolute value of the difference between the two potentials (=Vp21−Vp22)exceeds a threshold V_(Th4). Otherwise, as expressed by Equation 11, itis determined that no tilt has occurred.

|Vp 21−Vp 22|≦V _(Th) ₄  [Equation 11]

[0197] As such, after calculation of the potential difference Vp21between the tracking error signal at the header 1,2 field and thereference level, and the potential difference Vp22 between the trackingerror signal at the header 3,4 field and the reference level, thedifference between the two potential differences Vp21 and Vp22 iscompared with the threshold, as a result of which the magnitude and thedirection of tilt are detected.

[0198] When the difference between the two potential differences(Vp21−Vp22) is ΔV4, the absolute value of ΔV4 indicates the magnitude oftilt, the sign of ΔV4 indicating the direction of tilt. That is, it canbe detected whether the disc is bending up or down with respect to anormal state.

[0199] If the sign of ΔV4 is negative (−), tilt is to be compensated byΔV4 in the positive (+) direction; otherwise, if the sign of ΔV4 ispositive (+), tilt is to be compensated by ΔV4 in the negative (−)direction. That is, compensation for tilt has to be performed in such adirection as to equalize the two potential differences Vp21 and Vp22.

[0200] Because the values of the tracking error signals detected at theheader 1,2 field and the header 3,4 field are variable depending on thedisc, the ratio of the two signals is normalized as expressed byEquation 12. $\begin{matrix}{{\frac{{Vp21} - {Vp22}}{{Vp21} + {Vp22}}} \prec V_{Th4}} & \left\lbrack {{Equation}\quad 12} \right\rbrack\end{matrix}$

[0201] That is, if the Equation 12 is satisfied, it is determined thatno tilt has occurred, otherwise, it means that tilt has occurred, afterwhich the magnitude and the direction of tilt are detected from theabsolute value and the sign of (Vp21−Vp22), respectively.

[0202] In connection with this, the tilt detector 205 calculates ΔV4 inthe above-described manner and outputs to the servo controller 207 tilterror signals indicating the magnitude and the direction of tilt, whichcorrespond to the absolute value and the sign of ΔV4, respectively. Theservo controller 207 converts the tilt error signals to a trackingdriving signal and outputs the tracking driving signal to the tiltdriver 209.

[0203] The tilt driver 209 moves the disc or the optical pickup fordirect control of tilt based on the tilt driving signal, i.e., by themagnitude of tilt in the positive (+) or negative (−) direction. Thatis, tilt control is performed in such a manner that the potentialdifference Vp21=|V_(HD12)−V_(TE)| is in symmetric relation with thepotential difference Vp22=|V_(HD34)−V_(TE)|, or that |Vp21−Vp22|≦V_(Th4)is satisfied.

[0204]FIG. 16 is a flowchart of the above procedure.

[0205] Referring to FIG. 16, the step 601 sets an initial tilt offsetT0, a threshold V_(Th4) and a tilt limit iteration number NL2.

[0206] The step 602 measures Vp21 and Vp22 at tilt offset T0, thencalculating the difference between Vp21 and Vp22, i.e., ΔV4(=Vp21−Vp22). The step 603 determines whether the absolute value of ΔV4exceeds the threshold V_(Th4) preset in the step 601. If the absolutevalue of ΔV4 is smaller than or equal to the threshold V_(Th4) in thestep 603, which means that no tilt has occurred, the routine isterminated; otherwise, if the absolute value of ΔV4 is larger than thethreshold V_(Th4), which means that tilt has occurred, the step 604determines the direction of tilt from a judgment of whether the sign ofΔV4 is positive (+) or negative (−). With the negative (−) sign of ΔV4,the step 605 increments the tilt offset T0 and the step 606 measuresagain the variation ΔV4 of the tracking error signals detected at theheader 1,2 field and the header 3,4 field. Then, the step 607 comparesthe absolute value of the variation ΔV4 with the threshold V_(Th4). Ifthe absolute value of ΔV4 is smaller than or equal to the thresholdV_(Th4), the routine is terminated; otherwise, if the absolute value ofΔV4 is larger than the threshold V_(Th4), which means that tilt is notcompletely compensated, the procedure proceeds to the step 608. The step608 applies the current loop iteration number to the tilt iterationnumber and the step 609 compares the tilt iteration number with thepreset tilt limit iteration number NL2. If the tilt iteration number islarger than or equal to the preset tilt limit iteration number NL2, theroutine terminates; otherwise, the procedure returns to the step 605.

[0207] Meanwhile, if the sign of ΔV4 is positive (+) in step 604, thestep 610 decrements the tilt offset T0 and the above process is repeatedin steps 611 to 614.

[0208] Such as in the present invention, during tilt or servo control,the quantity of tilt between the optical axis and the disc plane can bedetected and controlled by any one of the above-stated methods.

[0209] The present invention presets the thresholds and reduces timerequired for detecting and compensating tilt during the actual datawrite operation, thereby enabling a real time write operation throughrapid stabilization of tracking servo.

Defocus Detection and Compensation

[0210] Using read channel 2 signal

[0211] The present invention detects defocus from variations of VFO1 andVFO3 signals in the header field. Here, the error area is used becauseof similarity of land and groove characteristics, and the reason forusing signals of VFO1 and VFO3 areas in the header field lies in thatthe VFO1 and VFO3 areas are the longest and most stable areas in theheader field and easy to detect.

[0212] To detect the magnitude and the direction of defocus fromvariations of read channel 2 signals detected at VFO1 and VFO3 areas inthe header field, read channel 2 signals among the error signalsdetected at the RF and servo error generator 203 are input to thedefocus detector 206. The defocus detector 206 detects the levels of theread channel 2 signals, i.e., peak-to-peak voltages Vpp31 and Vpp32 todetermine presence of defocus.

[0213] Table 5 shows read channel 2 signals detected at VFO1 and VFO3areas with track on after controlling defocus and detrack at tilt=0(i.e., mechanism=0) for obtaining highest and planarized tracking errorsignals, in which the read channel 2 signal levels at VFO1 and VFO3areas are changed due to variation of a defocus offset with fixed tiltand detrack offsets. TABLE 5 Defocus Offset [ ] VFO1 [V] VFO3 [V] 0.001.00 2.00 0.142 0.087 3.00 0.178 0.151 4.08 0.192 0.199 5.00 0.162 0.2016.00 0.119 0.181 7.00 0.064 0.139 8.00 0.021 0.089 9.00 10.00

[0214]FIG. 17 is a graph illustrating Table 5, in which no defocus isdetected at highest voltage levels Vpp31 and Vpp32 of read channel 2signals at VFO1 and VFO3 areas and the direction of defocus is detectedfrom the sign of Vpp31−Vpp32.

[0215] That is, detection of defocus is based on the principle thatvariations of read channel 2 signals at VFO1 and VFO3 areas in theheader field, e.g., peak-to-peak levels Vpp31 and Vpp32 depend on thedegree of defocus.

[0216] If no defocus has occurred, i.e., “on-focus”, then Vpp31−Vpp32≈0and Vpp31+Vpp32=maximum, where Vpp31 and Vpp32 are peak-to-peak voltagesof read channel 2 signals detected at VFO1 and VFO3 areas. No defocus isalso detected when Vpp31−Vpp32≦V_(Th5), where V_(Th5) is a predeterminedthreshold value, instead of Vpp31−Vpp32≈0.

[0217] Otherwise, if defocus has occurred, then Vpp31−Vpp32≠0 (the signof the difference value depends on the direction of defocus) andVpp31+Vpp32≠maximum. Defocus is also detected when Vpp31−Vpp32>V_(Th5),where V_(Th5) is a predetermined threshold value. The direction ofdefocus is known from the sign of Vpp31−Vpp32.

[0218] Referring to Table 5 or FIG. 17, at defocus offset 4.08,Vpp31−Vpp32≈0, Vpp31+Vpp32=maximum and the curve has an inflection. Thatis, the value Vpp31−Vpp32 is varying in one direction (e.g.,successively increasing or decreasing) on the basis of defocus offset4.08, which facilitates signal detection.

[0219]FIGS. 18a-18 c are exemplary diagrams showing the level variationof the read channel 2 signals detected at a variable defocus offsetunder the same conditions as Table 5.

[0220] Referring to FIGS. 18a-18 c, the value Vpp31+Vpp32 is at maximumin FIG. 18b, which means that no defocus has occurred. And, defocus isdetected in FIGS. 18a and 18 c.

[0221] If the value Vpp31−Vpp32 is ΔV5 and the absolute value of ΔV5 islarger than the threshold V_(Th5) or the value Vpp31+Vpp32 is not atmaximum, then compensation for defocus has to be performed in thepositive (+)/negative (−) direction when the sign of ΔV5 is negative(−)/positive (+).

[0222] As the values Vpp31 and Vpp32 are variable depending on the disc,the ratio of the two signals is normalized as expressed by Equation 13.$\begin{matrix}{{\frac{{Vpp31} - {Vpp32}}{{Vpp31} + {Vpp32}}} \prec V_{Th5}} & \left\lbrack {{Equation}\quad 13} \right\rbrack\end{matrix}$

[0223] If Equation 13 is satisfied, no defocus has occurred. Otherwise,if Equation 13 is not satisfied, it is determined that focus hasoccurred, and the magnitude and the direction of defocus are detectedfrom the absolute value and the sign of the difference (=Vpp31−Vpp32),respectively.

[0224] In connection with this, the defocus detector 206 outputs to theservo controller 207 defocus error signals indicating the magnitude andthe direction of defocus detected in the above process. The servocontroller 207 converts the defocus error signals to a focus drivingsignal and outputs the focus driving signal to the focus driver 210.

[0225] The tracking driver 210 drives a focus actuator in the opticalpickup 202 based on the focus driving signal, i.e., moves the opticalpickup 202 by the magnitude of defocus in the positive (+) or negative(−) direction such that the object lens is separated from the opticaldisc at a constant distance.

[0226]FIG. 19 is a flowchart of the above procedure.

[0227] Referring to FIG. 19, the step 701 sets an initial defocus offsetDF0, a threshold V_(Th5) and a defocus limit iteration number NL3. Thestep 702 measures Vpp31 and Vpp32 at defocus offset DF0, thencalculating the difference ΔV5 (=Vpp31−Vpp32) and the sum ΔV5′(=Vpp31+Vpp32). The step 703 determines whether the absolute value ofΔV5 is smaller than or equal to the threshold V_(Th5) preset in the step701 and the sum ΔV5′ is at maximum. If the absolute value of ΔV5 issmaller than or equal to the threshold V_(Th5) and the sum ΔV5′ is atmaximum in the step 703, which means that no defocus has occurred, theroutine is terminated. Otherwise, if the absolute value of ΔV5 is largerthan the threshold V_(Th5) or the sum ΔV5′ is not at maximum, whichmeans that defocus has occurred, the step 704 detects the sign of ΔV5.

[0228] With negative (−) sign of ΔV5, the step 705 increments thedefocus offset DF0 and the step 706 measures again the difference ΔV5and the sum ΔV5′ for the read channel 2 signals detected at VFO1 andVFO3 areas in the header field. Then, the step 707 compares the absolutevalue of ΔV5 with the threshold V_(Th5) and determines whether the sumΔV5′ is at maximum. If the absolute value of ΔV5 is smaller than orequal to the threshold V_(Th5) and the sum ΔV5′ is at maximum, theroutine is terminated. Otherwise, if the absolute value of ΔV5 is largerthan the threshold V_(Th5) or the sum ΔV5′ is not at maximum, whichmeans that defocus is not completely compensated, the procedure proceedsto the step 708. The step 708 applies the current loop iteration numberto the defocus iteration number and the step 709 compares the defocusiteration number with the preset defocus limit iteration number NL3.

[0229] If the defocus iteration number is larger than or equal to thepreset defocus limit iteration number, the routine terminates;otherwise, the procedure returns to the step 705.

[0230] Meanwhile, if the step 704 determines that the sign of ΔV5 ispositive (+), the defocus offset DF0 is decremented in step 710 and theabove process is repeated in steps 711 to 714. Likewise, thedetermination and comparison process for the defocus limit iterationnumber NL3 is the same in reason as the previously described process forthe detrack limit iteration number NL1.

[0231] 2) Using read channel 1 signal and read channel 2 signal

[0232] The present invention also detects defocus using both readchannel 1 signals and read channel 2 signals.

[0233] That is, defocus is detected from variation of read channel 1signals at VFO1 and VFO3 areas in the header field and the magnitude andthe direction of defocus are detected from variation of read channel 2signals.

[0234] For this purpose, among the error signals detected at the RF andservo error generator 203, both read channel 1 signals and read channel2 signals are input to the defocus detector 206. The defocus detector206 detects the levels of the read channel 1 signals, i.e., peak-to-peakvoltages to determine presence of defocus.

[0235] Table 6 shows read channel 1 signals detected at VFO1 and VFO3areas with track on after controlling defocus and detrack at tilt=0(i.e., mechanism=0) for obtaining highest and planarized tracking errorsignals, in which the read channel 1 signal levels at VFO1 and VFO3areas are changed due to variation of a defocus offset with fixed tiltand detrack offsets. TABLE 5 Defocus Offset [ ] VFO1 [V] VFO3 [V] 0.000.00 1.00 2.00 0.126 0.137 3.00 0.162 0.185 4.08 0.192 0.187 5.00 0.2060.183 6.00 0.176 0.153 7.00 0.121 0.114 8.00 0.069 0.073 9.00 10.00

[0236]FIG. 20 is a graph illustrating Table 6, in which no defocus isdetected at highest voltage levels Vpp31′ and Vpp32′ of read channel 1signals at VFO1 and VFO3 areas, and the magnitude and the direction ofdefocus are detected from the absolute value and the sign ofVpp31′−Vpp32′, respectively.

[0237] That is, detection of defocus is based on the principle thatvariations of read channel 1 signals at VFO1 and VFO3 areas in theheader field, e.g., peak-to-peak levels Vpp31′ and Vpp32′ depend on thedegree of defocus.

[0238] If no defocus has occurred, i.e., “on-focus”, thenVpp31′−Vpp32′≈0 and Vpp31′+Vpp32′=maximum, where Vpp31′ and Vpp32′ arepeak-to-peak voltages of read channel 1 signals detected at VFO1 andVFO3 areas.

[0239] No defocus is also detected when Vpp31′+Vpp32′=maximum andVpp31′−Vpp32′≦V_(Th55), where V_(Th55) is a predetermined thresholdvalue, instead of Vpp31′−Vpp32′≈0.

[0240] Otherwise, if defocus has occurred, then Vpp31′−Vpp32′≠0 (thesign of the difference value depends on the direction of defocus) andVpp31′+Vpp32′≠maximum. Defocus is also detected whenVpp31′−Vpp32′>V_(Th55), the direction of defocus being known from thesign of Vpp31′−Vpp32′.

[0241] Referring to Table 6 or FIG. 20, at defocus offset 4.08,Vpp31′−Vpp32′≈0, Vpp31′+Vpp32′=maximum.

[0242] For read channel 1 signals, it is necessary to define an activearea in order to acquire accurate detection of defocus because there aremany cases where Vpp31′−Vpp32′≦V_(Th55), as shown in FIG. 20.

[0243]FIG. 22a shows the value of Vpp31′−Vpp32′ for read channel 1signals and FIG. 22b shows the value of Vpp31−Vpp32 for read channel 2signals.

[0244] Referring to FIGS. 22a and 22 b, when no defocus has occurred,i.e., near the defocus offset of 4.08, Vpp31′−Vpp32′ for read channel 1signals and Vpp31−Vpp32 for read channel 2 signals are both changedgreatly. After the defocus offset of 4.08, Vpp31′−Vpp32′ for readchannel 1 signals decreases but Vpp31−Vpp32 for read channel 2 signalsis constant.

[0245] Thus the active area can be defined as an interval where bothVpp31′−Vpp32′ for read channel 1 signals and Vpp31−Vpp32 for readchannel 2 signals change proportionally.

[0246]FIGS. 21a-21 c are exemplary diagrams showing the level variationof the read channel 1 signals detected at a variable defocus offsetunder the same conditions as Table 6.

[0247] Referring to FIGS. 21a-21 c, the difference value Vpp31′−Vpp32′approaches zero in all cases and the sum Vpp31′+Vpp32′ is at maximum inFIG. 21b. Thus defocus is detected in FIGS. 21a and 21 c, no defocusbeing detected in FIG. 21b. That means, it is determined that defocushas occurred in FIGS. 21a and 21 c, where the difference valueVpp31′−Vpp32′ approaches zero in both cases and the non-active area isdefined.

[0248] If the value Vpp31′−Vpp32′ is ΔV5″ and the absolute value of ΔV5″is larger than the threshold V_(Th55), then compensation for defocus hasto be performed in the positive (+)/negative (−) direction when the signof ΔV5″ is negative (−)/positive (+). The magnitude of defocus can bemeasured as the size of the active area.

[0249] As the values Vpp31′ and Vpp32′ are variable depending on thedisc, the ratio of the two signals is normalized as expressed byEquation 14. $\begin{matrix}{{\frac{{Vpp31}^{\prime} - {Vpp32}^{\prime}}{{Vpp31}^{\prime} + {Vpp32}^{\prime}}} \prec V_{Th55}} & \left\lbrack {{Equation}\quad 14} \right\rbrack\end{matrix}$

[0250] If Equation 14 is satisfied, no defocus has occurred; otherwise,if Equation 14 is not satisfied, it is determined that focus hasoccurred. The direction of defocus is detected from the sign ofVpp31′−Vpp32′, the magnitude of defocus being detected from the size ofthe active area detected using both the read channel 1 and the readchannel 2.

[0251] In connection with this, the defocus detector 206 outputs to theservo controller 207 defocus error signals indicating the direction ofdefocus detected in the above process. The servo controller 207 convertsthe defocus error signals to a focus driving signal and outputs thefocus driving signal to the focus driver 210.

[0252] The tracking driver 210 drives a focus actuator in the opticalpickup 202 based on the focus driving signal, i.e., moves the opticalpickup 202 in the positive (+) or negative (−) direction such that theobject lens is separated from the optical disc at a constant distance.

[0253] As such, the present invention can detect the magnitude and thedirection of defocus and compensate for them using read channel 1signals or read channel 2 signals detected at VFO1 and VFO3 areas in theheader field.

[0254] The system, the present invention checks defocus at a pluralityof header fields predefined during initialization of the system in theabove-stated way and memories the magnitude and the direction of defocusat the corresponding position. Thus the present invention can compensatefor defocus according to the previously detected magnitude and directionof defocus at the corresponding position during an actual datawriting/reading operation and thereby rapidly stabilize focus servo.

[0255] Furthermore, the present invention capable of real-time feedbackcan detect defocus with all servo offsets, e.g., tracking and focusservo on and immediately compensate for defocus during an actual datawriting/reading operation.

[0256] 3) Using tracking error signal

[0257] The present invention also detects defocus using a leveldifference between tracking error signals detected at the header 1,2field and the header 3,4 field staggered with respect to each other anda reference signal. The reference signal level is the center level ofthe tracking error signal detected at a user area.

[0258] For this, the tracking error (TE) signals among the servo errorsignals detected at the RF and servo error generator 203 are input tothe defocus detector 206.

[0259] After sampling the tracking error signals output from the header1,2 field and the header 3,4 field, the defocus detector 206 detects thelevel difference between the tracking error signals and the referencesignal

[0260] Table 7 shows the tracking error signal levels under bestconditions for generating servo error signals while controlling defocusand defocus in a state of tilt zero (i.e., mechanism 0), in which thetracking error signal levels change depending on variation of a defocusoffset at fixed tilt and detrack offsets. TABLE 7 Defocus Offset [ ]Header 1, 2 [V] Header 3, 4 [V] 0.00 1.00 2.00 2.2 1.70 3.00 3.3 2.704.08 3.5 3.30 5.00 3.1 3.70 6.00 2.7 3.70 7.00 2.2 3.10 8.00 1.8 2.709.00 10.00

[0261]FIG. 23 is a graph illustrating Table 7, in which no defocusoccurs when the potential difference between the tracking error signaldetected at the header 1,2 field and the reference signal is insymmetric relation with the potential difference between the trackingerror signal detected at the header 3,4 field and the reference signal.

[0262] That is, the tracking error signals are significantly shifted upand down in the header field. For tracking error signals detected at theuser area on which data is actually written, the two potentialdifferences are almost equal to each other when no defocus occurs, i.e.,the beam is at the track center, whereas they are not equal to eachother when defocus occurs, i.e., the beam passes through the header 1,2field and the header 3,4 field.

[0263] Thus whether or not defocus has occurred can be determined bycomparing the potential difference between the tracking error signal atthe header 1,2 field and the reference signal (tracking error signalpotential at header 1,2 field−reference potential=Vp31) with thepotential difference between the tracking error signal at the header 3,4field and the reference signal (tracking error signal potential atheader 3,4 field−reference potential=Vp32).

[0264]FIGS. 24a-24 c are exemplary diagrams showing tracking errorsignals detected at a variable defocus offset with tracking and focuson.

[0265] Referring FIGS. 24a-24 c, the left-hand signal is the trackingerror signal V_(HD12) detected at the header 1,2 field, the right-handsignal being the tracking error signal V_(HD34). A voltage V_(TE)detected at the center level of the tracking error signal at the userarea is preferably the voltage of the reference level.

[0266] In a case where no defocus occurs, the potential differencebetween the tracking error signal at the header 1,2 field and thereference level (Vp31=|V_(HD12)−V_(TE)|) is almost equal to thepotential difference between the tracking error signal at the header 3,4field and the reference level (Vp32=|V_(HD34)−V_(TE)|), as shown in FIG.24b. That is, the potential difference Vp31=|V_(HD12)−V_(TE)| is insymmetric relation with the potential difference Vp32=|V_(HD34)−V_(TE)|.

[0267] This can be expressed by Equation 15.

|V _(HD12) −V _(TE) |≈V _(HD34) −V _(TE)|  [Equation 15]

[0268] It is determined that no defocus has occurred, when the potentialdifference Vp31 between the tracking error signal at the header 1,2field and the reference level is not equal to the potential differenceVp32 between the tracking error signal at the header 3,4 field and thereference level, as shown in FIGS. 24a and 24 c, i.e., the potentialdifference Vp31 is in asymmetric relation with to the potentialdifference Vp32. The asymmetry increases with greater magnitude ofdefocus.

[0269] Thus it is determined that defocus has occurred, when thepotential difference between the tracking error signal at the header 1,2field and the reference level (Vp31=|V_(HD12)−V_(TE)|) is not equal tothe potential difference between the tracking error signal at the header3,4 field and the reference level (Vp32=|V_(HD34)−V_(TE)|), i.e., theabsolute value of the difference between the two potentials (=Vp31−Vp32)exceeds a threshold V_(Th6). Otherwise, as expressed by Equation 16, itis determined that no defocus has occurred.

|Vp 31−Vp 32|≦V _(Th6)  [Equation 16]

[0270] As such, after calculation of the potential difference Vp31between the tracking error signal at the header 1,2 field and thereference level, and the potential difference Vp32 between the trackingerror signal at the header 3,4 field and the reference level, thedifference between the two potential differences Vp31 and Vp32 iscompared with the threshold, as a result of which the magnitude and thedirection of defocus are detected.

[0271] When the difference between the two potential differences(Vp31−Vp32) is ΔV6, the absolute value of ΔV6 indicates the magnitude ofdefocus, the sign of ΔV6 indicating the direction of defocus.

[0272] If the sign of ΔV6 is negative (−), defocus is to be compensatedby ΔV6 in the positive (+) direction; otherwise, if the sign of ΔV6 ispositive (+), defocus is to be compensated by ΔV6 in the negative (−)direction. That is, compensation for defocus has to be performed in sucha direction as to equalize the two potential differences Vp31 and Vp32.

[0273] Because the values of the tracking error signals detected at theheader 1,2 field and the header 3,4 field are variable depending on thedisc, the ratio of the two signals is normalized.

[0274] In connection with this, the defocus detector 206 calculates ΔV6in the above-described manner and outputs to the servo controller 207defocus error signals indicating the magnitude and the direction ofdefocus, which correspond to the absolute value and the sign of ΔV6,respectively. The servo controller 207 converts the defocus errorsignals to a focus driving signal and outputs the focus driving signalto the focus driver 210.

[0275] The focus driver 208 moves a focus actuator in the optical pickup202 based on the focus error signal so that the object lens is separatedfrom the optical disc at a constant distance. That is, defocus iscontrolled in such a manner that the potential difference between thetracking error signal at the header 1,2 field and the reference level(Vp31=|V_(HD12)−V_(TE)|) is in symmetric relation with the potentialdifference between the tracking error signal at the header 3,4 field andthe reference level (Vp32=|V_(HD34)−V_(TE)|), or the relationship|Vp31−Vp32|≦V_(Th6) is satisfied.

[0276]FIG. 25 is a flowchart of the above procedure.

[0277] Referring to FIG. 25, the step 801 sets an initial defocus offsetDF0, a threshold V_(Th6) and a defocus limit iteration number NL3.

[0278] The step 802 measures Vp31 and Vp32 at defocus offset DF0, thencalculating the difference between Vp31 and Vp32, i.e., ΔV6(=Vp31−Vp32). The step 803 determines whether the absolute value of ΔV6exceeds the threshold V_(Th6) preset in the step 801. If the absolutevalue of ΔV6 is smaller than or equal to the threshold V_(Th6) in thestep 803, which means no defocus, the routine is terminated; otherwise,if the absolute value of ΔV6 is larger than the threshold V_(Th6), whichmeans that defocus has occurred, the step 804 determines the directionof defocus from a judgment of whether the sign of ΔV6 is positive (+) ornegative (−).

[0279] With the negative (−) sign of ΔV6, the step 805 increments thedefocus offset DF0 and the step 806 measures again the variation ΔV6 ofthe tracking error signals detected at the header 1,2 field and theheader 3,4 field. Then, the step 807 compares the absolute value of thevariation ΔV6 with the threshold V_(Th6). If the absolute value of ΔV6is smaller than or equal to the threshold V_(Th6), the routine isterminated; otherwise, if the absolute value of ΔV6 is larger than thethreshold V_(Th6), which means that defocus is not completelycompensated, the procedure proceeds to the step 808. The step 808applies the current loop iteration number to the defocus iterationnumber and the step 809 compares the defocus iteration number with thepreset defocus limit iteration number NL3. If the defocus iterationnumber is larger than or equal to the preset defocus limit iterationnumber NL3, the routine terminates; otherwise, the procedure returns tothe step 805.

[0280] Meanwhile, if the sign of ΔV6 is positive (+) in step 804, thestep 810 decrements the defocus offset DF0 and the above process isrepeated in steps 811 to 814.

[0281] The present invention presets the thresholds and reduces timerequired for detecting and compensating defocus during an actual datawriting operation, thereby enabling a real time writing operationthrough rapid stabilization of focus servo.

[0282] According to the present invention, the detrack limit iterationnumber NL1, the tilt limit iteration number NL2 and the defocus limititeration number NL3 are determined in an apriori manner and may bealtered by the designer.

[0283] In a case where the direction is not detected during detection ofdetrack, tilt and defocus by the above methods, detrack, tilt anddefocus can be compensated by the variations obtained in the respectivedetecting processes. For this, the respective offsets have to becontrolled such that the variations become smaller. For example, if thevariations at the current offsets are smaller than those at the previousoffsets, it is necessary to check whether the variations are within therange of predetermined thresholds, while controlling the offsets in thesame direction (i.e., positive (+) or negative (−) direction).Otherwise, if the variations at the current offsets are increased fromthose at the previous offsets, it is necessary to check whether thevariation are within the range of predetermined thresholds, whilecontrolling the offsets in the reverse direction. This procedure isrepeated until the variations are within the predetermined thresholds,and then the control is terminates.

[0284] Although it is assumed in the above description of the presentinvention that the procedure for detecting and compensating each ofdetrack, tilt and defocus is performed under the condition the others donot occur, actually, all of detract, tilt and defocus or any one of themmay occur. It is thus preferable to sequentially perform all thecompensation processes for detrack, tilt and defocus. There is noparticular limitation on the order of the processes and may be varieddepending on the designer. In an embodiment of the present invention, asillustrated in FIG. 26, the controls are performed in the order ofdetrack, tilt and defocus.

[0285] The above controls as illustrated in FIG. 26 may be performedonce in initializing the system, or periodically during arecording/reproducing operation, or at any time that the controller suchas microcomputer affords to its. Also, the controls may be performedduring the initialization of the system or while the system is running.

[0286] In performing the controls during the initialization of thesystem, the magnitude and the direction of detrack, tilt and defocus aredetected at one or more predefined positions in inner or outercircumferences, followed by a recording or reproducing operation, or themagnitude and the direction of detrack, tilt and defocus detected at thepredetermined positions are stored and then immediately used for thecontrols during the actual running operation.

[0287] The method for reproducing records for the optical recordingmedium according to the present invention has the following advantagesin that: (1) the magnitude and the direction of detrack, tilt anddefocus can be detected from read channel 2 signals detected at theheader fields staggered on the basis of the tract center, or trackingerror signals, and compensates for detrack, tilt and defocus, therebypreventing deterioration of data quality caused by detrack, tilt anddefocus during a recording/reproducing operation and enabling the stableoperation of the system; (2) tilt can be detected in a stable andaccurate manner without using a separate light-receiving element in ahigh-density optical disc; and (3) the focus servo is rapidly stabilizedto enable real-time recording as well as the stable operation of thesystem.

[0288] While the invention has been shown and described with referenceto a certain preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method for reproducing records for opticalrecording medium -which has a plurality of non-writable areas ofdifferent phases between writable data areas containing wobbledinformation on a track for recognizing a reference frequency todistinguish profiles of the data areas, the method comprising the stepsof: (a) determining a difference between a difference signal of opticalreflecting signals of the optical recording medium detected at thenon-writable area and the center level at an adjacent data area tooutput a first signal; (b) determining a difference between a differencesignal of optical reflecting signals of the optical recording mediumdetected at a second non-writable area and the center level at anadjacent area to output a second signal, the second non-writable areabeing different in phase from the non-writable area; (c) determining adifference between the first signal and the second signal to output avariation; (d) comparing the variation with a predetermined threshold,determining that detrack has occurred, if the variation exceeds thethreshold, and outputting the resulting value; and (e) performing atracking servo based on the resulting value.
 2. The method as claimed inclaim 1, wherein the difference signal between the optical reflectingsignals includes a read channel 2 signal generated from electricalsignals output in proportion to the quantity of beam reflected from theoptical recording medium.
 3. The method as claimed in claim 2, whereinthe center level at the data area includes a center voltage of a wobblesignal detected at the data area.
 4. The method as claimed in claim 2,wherein the steps (a) and (b) of generating the first and second signalsuse only a defined part of the non-writable areas.
 5. The method asclaimed in claim 4, wherein the steps (a) and (b) of generating thefirst and second signals use variable frequency oscillator (VFO) areasof the non-writable areas.
 6. The method as claimed in claim 2, whereinin the steps (a) and (b) of generating the first and second signals, thefirst signal is a potential difference between the read channel 2 signaldetected at the non-writable area and a wobble center level, and thesecond signal is a potential difference between the read channel 2signal detected at the second non-writable area and the wobble centerlevel at the adjacent data area, the second non-writable area beingdifferent in phase from the non-writable area.
 7. The method as claimedin claim 1, wherein the difference signal between the optical reflectingsignals includes a tracking error signal obtained by filtering the readchannel 2 signal generated from electrical signals output in proportionto the quantity of beam reflected from the optical recordng medium. 8.The method as claimed in claim 7, wherein the center level at the dataarea includes the center level of the tracking error signal detected atthe data area.
 9. The method as claimed in claim 7, wherein in the steps(a) and (b) of generating the first and second signals, the first signalis a potential difference between the tracking error signal detected atthe non-writable area and the center level of the tracking error signalof the adjacent data area, and the second signal is a potentialdifference between the tracking error signal detected at the secondnon-writable area and the center level of the tracking error signal, thesecond non-writable area being different in phase from the non-writablearea.
 10. The method as claimed in claim 1, wherein the detrackdetermining step (d) determines as on-track where the beam is focused ona track center, if the variation does not exceed the threshold, andoutputs the resulting value.
 11. The method as claimed in claim 1,wherein the tracking servo step (e) detects the magnitude and thedirection of detrack from the value and the sign of the variation,respectively.
 12. The method as claimed in claim 1, wherein the trackingservo step (e) performs the tracking servo in such a manner as toequalize the level of the first signal to the level of the secondsignal.
 13. The method as claimed in claim 1, wherein the tracking servostep (e) performs the tracking servo in such a manner that two trackingerror signals of different phases is in symmetric relation with eachother with respect to the center level of the adjacent data area.
 14. Amethod for reproducing records for optical recording medium which has aplurality of non-writable areas of different phases between writabledata areas containing wobbled information on a track for recognizing areference frequency to distinguish profiles of the data areas, themethod comprising the steps of: (a) determining a difference signalbetween optical reflecting signals each detected at the pluralnon-writable areas of different phases to output a variation; (b)comparing the variation with a predetermined threshold, determining thatdefocus has occurred, if the variation exceeds the threshold, andoutputting the resulting value; and (c) performing a focus servo basedon the resulting value.
 15. The method as claimed in claim 14, whereinin the variation outputting step (a), the optical reflecting signalsdetected at the non-writable areas include read channel 2 signalsgenerated from electrical signals output in proportion to the quantityof beam reflected from the optical recording medium.
 16. The method asclaimed in claim 14, wherein in the variation outputting step (a), theoptical reflecting signals detected at the non-writable areas includeread channel 1 signals generated from electrical signals output inproportion to the quantity of beam reflected from the optical recordingmedium.
 17. The method as claimed in claim 14, wherein in the variationoutputting step (a), a peak-to-peak voltage of read channel 1 signals orread channel 2 signals detected at the non-writable areas is a firstsignal, and a peak-to-peak voltage of read channel 1 signals or readchannel 2 signals detected at a second non-writable areas is a secondsignal, the second non-writable areas being different in phase from thenon-writable areas, the variation being the difference between the firstsignal and the second signal.
 18. An apparatus for reproducing recordsfor optical recording medium which has a plurality of non-writable areasof different phases between writable data areas containing wobbledinformation on a track for recognizing a reference frequency todistinguish profiles of the data areas, the apparatus comprising: asignal generator for generating a difference signal between opticalreflecting signals from electrical signals generated from an opticalpickup for recording/reproducing information on/from the opticalrecording medium; a detrack detector for detecting detrack of theoptical recording medium from a variation of the difference signalbetween the optical reflecting signals of the non-writable areas outputfrom the signal generator, and outputting a detrack error signal; a tiltdetector for detecting tilt of the optical recording medium from avariation of the difference signal between the optical reflectingsignals of the non-writable areas output from the signal generator, andoutputting a tilt error signal; a defocus detector for detecting defocusof the optical recording medium from a variation of the differencesignal between the optical reflecting signals of the non-writable areasoutput from the signal generator, and outputting a defocus error signal;a servo controller for generating a tracking driving signal from thedetrack error signal detected at the detract detector, a tilt drivingsignal from the tilt error signal detected at the tilt detector, and afocus driving signal from the defocus error signal detected at thedefocus detector; a tracking driver for controlling the optical pickupbased on the tracking driving signal to compensate for detrack; a tiltdriver for controlling the optical pickup based on the tilt drivingsignal to compensate for tilt; and a focus driver for controlling theoptical pickup based on the focus driving signal to compensate fordefocus.
 19. The apparatus as claimed in claim 18, wherein thedifference signal between the optical reflecting signals includes a readchannel 2 signal or a tracking error signal.
 20. The apparatus asclaimed in claim 18, wherein the servo controller generates the trackingdriving signal, the tilt driving signal and the focus driving signalwhile changing each offset in such a manner that the variables detectedat the detrack detector, the tilt detector and the defocus detectordecrease.